Microbial Larvicide Application by a Large-Scale, Community-Based Program Reduces Malaria Infection Prevalence in Urban Dar Es Salaam, Tanzania

Abstract

BackgroundMalaria control in Africa is most tractable in urban settlements yet most research has focused on rural settings. Elimination of malaria transmission from urban areas may require larval control strategies that complement adult mosquito control using insecticide-treated nets or houses, particularly where vectors feed outdoors.Methods and FindingsMicrobial larvicide (Bacillus thuringiensis var. israelensis (Bti)) was applied weekly through programmatic, non-randomized community-based, but vertically managed, delivery systems in urban Dar es Salaam, Tanzania. Continuous, randomized cluster sampling of malaria infection prevalence and non-random programmatic surveillance of entomological inoculation rate (EIR) respectively constituted the primary and secondary outcomes surveyed within a population of approximately 612,000 residents in 15 fully urban wards covering 55 km2. Bti application for one year in 3 of those wards (17 km2 with 128,000 residents) reduced crude annual transmission estimates (Relative EIR [95% Confidence Interval] = 0.683 [0.491–0.952], P = 0.024) but program effectiveness peaked between July and September (Relative EIR [CI] = 0.354 [0.193 to 0.650], P = 0.001) when 45% (9/20) of directly observed transmission events occurred. Larviciding reduced malaria infection risk among children ≤5 years of age (OR [CI] = 0.284 [0.101 to 0.801], P = 0.017) and provided protection at least as good as personal use of an insecticide treated net (OR [CI] = 0.764 [0.614–0.951], P = 0.016).ConclusionsIn this context, larviciding reduced malaria prevalence and complemented existing protection provided by insecticide-treated nets. Larviciding may represent a useful option for integrated vector management in Africa, particularly in its rapidly growing urban centres.

Is Mosquito Larval Source Management Appropriate for Reducing Malaria in Areas of Extensive Flooding in The Gambia? A Cross-over Intervention Trial

“…The impact of larviciding on adult vectors in this study was limited. This finding is in marked contrast to studies in urban Tanzania and lowland and highland Kenya where LSM reduced exposure to transmission by 65-93% 13,20,21 and was associated with a 68-72% reduction in new parasite infections.…”

“…Larval control with microbial larvicides is effective in areas with relatively few well-defined habitats 13,14,20,21 but there has been no detailed evaluation of this method in areas with extensive habitats. This is the first large-scale assessment of the impact of larviciding in a rural ecosystem that is dominated by large areas of flooding.…”

“…The recent successes that have been achieved with LSM in rural and urban settings in east Africa 13,21 are clearly associated with differences in the transmission setting and habitat characteristics compared with the extensive floodplains of the Gambia River. Significant control of vectors can be achieved by hand application of larvicide where malaria transmission is focal and when water bodies are defined and accessible, where the water is stagnant and the flight range of the adult vector is not more than several kilometers.…”

“…This pilot trial was part of a series of studies to assess the feasibility of LSM and its impact on malaria morbidity in different eco-epidemiological settings (rural towns, 20 highland valleys, 13 desert fringe, 14 urban areas 16,21 ). The aim of this study was to evaluate the impact of LSM using microbial larvicides on vector populations and malaria incidence in the extensive seasonally flooded areas of the lower reaches of the river in rural Gambia.…”

New approaches for integrated and cost-effective malaria vector control

“…Biological larvicides such as Bacillus thuringiensis israelensis (Bti) are environmentally safe, target species specific in their mode of action, and show no resistances 12 . Within the last decade there has been a large body of evidence generated, showing that Bti based larviciding can bring down malaria vector populations by order of magnitude and significantly reduce malaria prevalence [13][14][15][16] . Recommendations from the WHO currently limit the use of larviciding to urban environments due to the high population density, which provides lower per capita intervention costs compared to less densely populated areas 4 .…”